Free androgen index (FAI)
HormonesAlso known as: androgen index, FAI, free testosterone index
FAI estimates the amount of active testosterone in the blood relative to binding proteins.
Reference range
Female, 18–29
| Level | Range |
|---|---|
| Normal | 0.4 to 8.4 |
Male, 18–29
| Level | Range |
|---|---|
| Normal | 30 to 128 |
Female, 30–39
| Level | Range |
|---|---|
| Normal | 0.4 to 8.4 |
Male, 30–39
| Level | Range |
|---|---|
| Normal | 24 to 122 |
Female, 40–49
| Level | Range |
|---|---|
| Normal | 0.4 to 8.4 |
Male, 40–49
| Level | Range |
|---|---|
| Normal | 14 to 126 |
Female, 50–59
| Level | Range |
|---|---|
| Normal | 0.4 to 6.6 |
Male, 50–59
| Level | Range |
|---|---|
| Normal | 18 to 82 |
Female, 60–69
| Level | Range |
|---|---|
| Normal | 0.4 to 6.6 |
Male, 60–69
| Level | Range |
|---|---|
| Normal | 18 to 82 |
Female, Over 70
| Level | Range |
|---|---|
| Normal | 0.4 to 6.6 |
Male, Over 70
| Level | Range |
|---|---|
| Normal | 18 to 82 |
Source: Ahead Health benchmark
* Reference ranges may vary between labs and assays. Always interpret results with your healthcare provider.
Why this matters
FAI provides a more accurate assessment of androgen status than total testosterone alone, especially when SHBG levels are abnormal. In women, elevated FAI can indicate polycystic ovary syndrome (PCOS), helping explain symptoms like irregular periods, hirsutism, acne, and fertility issues. In men, low FAI may reveal functional testosterone deficiency even when total levels appear normal, particularly in obesity, aging, or metabolic syndrome where SHBG changes mask true hormone availability. The index is especially valuable for detecting subtle hormonal imbalances that contribute to sexual dysfunction, mood changes, muscle loss, or metabolic problems, guiding more targeted hormone optimization strategies.
How this connects to other biomarkers
- Free Androgen Index (FAI = Testosteron, total × 100 / Sex hormone-binding globulin (SHBG)) approximates how much testosterone is biologically available.
- Age- and sex-stratified reference ranges per LabCorp (https://www.labcorp.com/tests/146688/free-androgen-index-fai): will not be displayed for men (no reference range for men); females roughly 0.4–8.4 (narrowing slightly to 0.4–6.6 after 49). Stop biotin supplementation ≥72h before the draw — high-dose biotin can interfere with this assay.
- Clinically, in women a FAI > 4.5 — even if still inside LabCorp's wider normal band — combined with elevated HOMA-Index, low SHBG, and LH/FSH Ratio > 2, is a primary diagnostic marker for Polycystic Ovary Syndrome (PCOS); don't rely on the band color alone to catch this.
- In men, low FAI with symptoms of low testosterone warrants further workup with Luteinizing hormone (Lutropin) and FSH, Follitropin even when total testosterone is normal.
How often should I test Free androgen index (FAI)?
The Free Androgen Index is reassessed whenever total testosterone and SHBG are measured together. For most adults that's once a year, or at 6 to 12 weeks after starting or adjusting hormone therapy.
At baseline / for screening: Once every 12 months from age 30 as part of a hormone panel, drawn in the morning between 7 and 10 AM when testosterone peaks.
When monitoring an intervention or change: Retest 6 to 12 weeks after starting or adjusting testosterone replacement therapy (gel, injection, pellet) to confirm you've reached the target range. Once stable on a dose, retest every 6 to 12 months. After meaningful body composition changes (weight loss, training, sleep improvement), expect 3 to 6 months for natural testosterone to shift.
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